Definition of Hydrachnellae

: a superfamily or higher group of Acarina comprising freshwater and marine mites that are usually rather large and often bright red, that have two pairs of eyes, tarsi usually with two claws and without an empodium, and chelicerae with a sickle-shaped movable digit, and that with few exceptions breathe by means of a well-developed tracheal system — see halacaridae , hydrachnidae